This invention relates to the preparation of stannic oxide. More particularly, this invention relates to a method for preparing stannic oxide of a type suitable for use as a polishing material for optical lenses and other light transmitting media formed from synthetic resins. Heretofore it has been difficult to reproducibly obtain a grade of stannic oxide with the properties that make it a desirable polishing agent for optically clear plastic materials. Too large a particle size or particles exhibiting dendritic or acicular projections usually cause excessive abrasion and scratching of the relatively soft lens surface. If the average particle size is too small, the particles may not be sufficiently abrasive to provide the desired polishing action. Even if the particle size is within the desired range, the shape of the particle, its hardness or both may cause an excessive amount of scratching.
It has now been found that by controlling certain process parameters during precipitation of hydrated stannic oxide and the subsequent calcining required to convert the oxide to the anhydrous form it is possible to reproducibly obtain a material which is a suitable lens polishing agent.